The present invention relates to a concrete vibrator for deaeration.
A concrete vibrator for us in concrete placing comprises, for example, a vibrator body A composed of a vibrating section I with a built-in exciter and a drive motor section II and a driving circuit box B housing therein a start switch, etc. and connected to the vibrator body by means of a cable or a sleeve. The concrete vibrator for sufficient deaeration of concrete by vibrating it with the vibrator body A pressed thereinto makes the concrete high-density and is indispensable for constructing high strength concrete buildings.
Incidentally, the primary performance that is required of the concrete vibrator is as follows: (1) No appreciable lowering of the vibration frequency and consequently no substantial decrease in the workload when the vibrator body is pressed into concrete; (2) Variable setting of the vibration frequency with the hardness of concrete, for example, its water content, for the most efficient deaeration; (3) Small lightweight, simple, trouble-free and easily movable and operable structure by a small number of operators, for instance, one operator; and (4) No special power supply is needed, but operable from even a power supply of large voltage fluctuation. Almost all of these requirements are determined by characteristics of a motor for driving an exciter for vibration, for instance, an eccentric pendulum. A high-speed drive motor that has been widely employed as a driving source in the past, such as an induction motor or commutator motor, has advantages and disadvantages cannot meet all of the requirements (1) to (4).
For example, the induction motor as a drive source is simple-structured, sturdy and robust against overload and operable from an ordinary commercial power supply, but this motor has a slip, and hence varies its rotation speed with load fluctuations, inevitably causing a substantial change in the vibration frequency. For instance, when the vibrator is pushed into concrete after starting the induction motor in the air, its speed of rotation drops, for example, from 12 krpm to about 6 krpm. Therefore, the use of the induction motor has the disadvantages of consumption of much time for required deaeration, a marked reduction of the workload and the likelihood of insufficient deaeration. Such disadvantages are also occasioned according to the hardness of concrete. If a motor of a large capacity is employed with a view to avoiding such defects, then the concrete vibrator will become bulky, heavy and expensive.
According to a conventionally popular system which drives the induction motor at a high frequency, for example, at around 200 Hz through use of a frequency converter such as a motor generator static type inverter, the workload can be increased by increasing the vibration frequency on one hand, but, on the other hand, the use of the frequency converter makes the concrete vibrator costly and heavy. In addition, the high-frequency driving causes an increase in iron loss, leading to appreciable heat generation by both of the rotor and the stator. Accordingly, there is a fear of burning of the motor especially when the concrete vibrator is driven for a long time in the air where the load on the vibrator is small, or when it is used for deaeration for a long time; a thermal protector needs to be provided for thermal protection of the motor. This is another disadvantage. In addition, because of the induction motor, it is necessary to use a motor of a large capacity for avoiding the afore-mentioned defect which results from a slip. Such a motor is also large and heavy. Moreover, the rotation speed of conventional induction motors is fixed, and hence cannot be varied with the hardness of concrete, besides the use of an engine generator of large voltage fluctuations is disadvantageous because of the afore-mentioned defects peculiar to the induction motor, such as great restrictions on the power supply that can be used.
Recently there has been made an attempt to employ, as the driving source for the exciter, a semiconductor motor with a sensor, also called a DC brushless motor, in place of the induction motor. The so-called DC brushless motor comprises a permanent magnet type rotor, a stator and a semiconductor element circuit including a magnetosensitive element for sensing the rotational position of the permanent magnet rotor in place of the brush of a DC motor, such as a Hall element, and a transistor as a substitute for the commutator.
This motor possesses advantages in its characteristics over the induction motor in that it is small, high efficiency (10 to 20% higher), excellent in controllability such as constant speed control and variable speed control, and in start characteristic and easy of maintenance because of the no-brush structure; therefore, this motor is free from the afore-noted defects of the induction motor. In this motor, however, the Hall element for sensing the rotational position of the permanent magnet rotor, an encoder, an inductance, etc. are readily damaged by vibration of the vibrator and heat generation from the motor. Accordingly, this motor suffers frequent occurrences of troubles and involves cumbersome maintenance, resulting in low utilization efficiency. To avoid this, it has been proposed to separate the motor drive circuit from the vibrator body containing the motor, the Hall element and the exciter and interconnect them via a cable. With this method, however, the distance allowable between the vibrator body and the drive circuit is a maximum of only 1.5 meters or so due to restrictions such as a noise trouble. Therefore, this method is not suitable for application to the concrete vibrator which requires an about 20 meters long cable for movement of the vibrator body and, further, this method needs a thick cable containing around eleven control lines including a power line for the motor, causing inconvenience to movement of the vibrator body. Thus, the application of the semiconductor motor with a ensor to the concrete vibrator introduces new problems which have not been encountered in the prior art.